DE3644866A1 - Sensor - Google Patents

Abstract

A sensor for converting a physical quantity into an electrical output signal comprises a light source from which a beam of light rays (60) is coupled into the end face of a light-conducting body. The light rays (60) are totally reflected or coupled out of the body at an interface (19) of the body in dependence on the physical quantity. The totally reflected light rays impinge on a second end face. A plurality of light-sensitive elements (22) is provided for detecting an angular range ( beta ) assumed by the beam after completed total reflection or coupling-out. To make the sensor adaptable to a multiplicity of applications and to generate reproducible digitised output values independently of local interference and long-time phenomena, the body is constructed as an elongated optical waveguide (59) in which light rays (60) are totally reflected a number of times, the optical index of refraction (n) of the optical waveguide (59) decreasing over its length towards the end face. The light-sensitive elements (22) are arranged at an axial distance from an end face. They form an impingement surface for a beam of light rays emerging from the end face. The elements are connected in an evaluating circuit comprising a counter which outputs the number of elements illuminated by the beam as output signal in digital form. <IMAGE>

Description

The invention relates to a sensor for converting a physical quantity into an electrical output signal, with a light source, from which a bundle of light rays into one light-guiding body is coupled, the light radiate at an interface of the body depending totally reflected from the physical size or from the Bodies are decoupled and the total reflected light radiate falling on an end face, and with a plurality photosensitive elements for detecting one of them  Bundle after total reflection or decoupling occupied angular range.

Such a sensor is from Patents abstracts of Japan, June 21, 1980, 55-53 878 (A).

The known sensor is used to measure the concentration of Lead accumulator battery electrolytes. He has one Light source on, from which a diverging beam through a cover on an oblique side surface of a pris matic, light-guiding body falls. A lower, long stretched interface of the body borders on the measure the electrolyte. The light rays of the divergent Rays falling on the lower interface are there, depending on their angle of incidence and the density of the electrolyte either totally reflected or but decoupled from the body into the electrolyte. The totally reflected light rays fall on another, also inclined interface of the prismatic body, on which there is a gate of photosensitive elements located. Depending on how big the density of the measured Is electrolyte, the boundary between the shifts still totally reflected and the decoupled light rays of the bundle on the lower interface with the electrolyte and thus also the corresponding boundary line that on the Gate of light-sensitive elements covered by total reflection beams of light. Because even when rays of light be coupled out, a partial beam at the lower boundary surface che is reflected, results over the length of the gate Seen the light-sensitive elements, an intensi the course of the disease from a relatively low signal value jumps to a relatively high signal value  increases. The known sensor now measures the amplitude of the the various elements of the gate falling light beam len and pulls the respective position of the jump-like Signal rise on the gate as a measure of the density of the Electrolytes.

However, the known sensor has the disadvantage that it is very high precision must be adjusted, because the only one total reflection of the light rays in the prismatic Body already slight misalignments of the incident Bundle of light rays to falsify the Lead measurement result. In addition, the known sensor has the Disadvantage that a measurement of the density of the electrolyte practically only in a punctiform manner, namely in one in the Practice very small length section of the interface in which the transition from total reflection to decoupling vari iert, so that the measurement result is only characteristic of the condition of the electrolyte in a container, e.g. one Total battery is when the electrolyte is in the battery has the same density uniformly. This is in practice, however, not always the case, because at a lighter one, e.g. warmer electrolyte components at the top of the Accumulate accumulator while heavier parts are below discontinue, on the other hand with accumulators, the movement subject to conditions, such as in motor vehicles the case is also due to the movement of the accumulators spatial density fluctuations occur to a considerable extent can. The known sensor cannot measure the acid density between the plates of a battery, and also installation in pipes is difficult. Another disadvantage of known sensor is that it is systematic for only one only measurement task, namely the measurement of the density of the  the medium adjacent to the prismatic body is suitable. Finally, the known sensor has the disadvantage that the measurement result is in analog form because the respective lige signal intensity at the individual elements of the gate is detected. The measurement result is therefore sensitive to All kinds of drift phenomena, for example aging Apparitions of the optical elements involved.

Numerous sensors are also known which are based on opto electronically using the refractive properties properties of a light guide physical quantities in electri Convert cal output signals, these sensors work but also all with analog measurement generation, only with known sensors for detecting fill levels it is known to exceed a single be agreed limit value to display as a digital yes / no signal gene, is a continuous digital level measurement however, this is also not possible.

From DE-OS 34 33 343 an optical measuring arrangement with fiber optic light transmission known. With this arrangement light becomes a light source into an optical fiber coupled in and occurs again at a radial interface out. The one emerging in the form of a conical beam of light Light strikes a mirrored reference surface at a distance to be measured from the face of the light head. That from the reference surface in turn reflected light hits insichreflectors that cause that the light is guided back in the same way is consequently back into the end face of the light guide arrives and after suitable decoupling from the transmission light Light detector is supplied. The measuring effect with this be  arrangement is based on the beam fanning, which is due to the dependence on the observation angle Light reflection of the insichreflector. The application of the known arrangement is therefore based on the length measurement a configuration of the type described limited.

From DE-OS 34 28 453 is also a sensor device known in which a transparent prism with two 90 ° mutually offset interfaces in a measuring medium immersed. If the measuring medium has a first predetermined density has, then a by means of a first light guide the first interface of light radiated at an oblique angle beam totally reflected there, reaches the second border area and is returned to a second receiving light guide reflected. Now the density of the measuring medium increases, so the light beam strikes the first one as soon as it occurs Interface of the prism into the medium and at the reception a zero signal is present on the light guide. This well-known sensor device is therefore only a yes / no signal generator settable, is a continuous digitized measurement not possible.

Another sensor is known from DE-OS 34 03 887, at an elongated, one-sided clamped light guide can be deflected by a gas flow. Towards the free A flat detector is located at the end of the light guide in the form of a luminescent body. Depending on the out steering of the light guide hits that which it emits Light on another surface area of the flat luminescent body on which on one or on a detector is arranged at both ends. From the intensity of the detector signal or from the quotient of the intensi  actions of both detector signals can now be the point of impact of the Light and thus indirectly the deflection of the light guide and from it the speed or the amount of deflecting gas can be determined. This sensor too therefore only allows analog measurements and it is open again a special application of gas velocity measurement limited or gas flow rates.

DE-OS 20 34 344 is a device for measurement physical quantities by measuring the intensity of a Light beam known. In this well-known Einrich one also takes advantage of the fact that Beam out light at the interfaces of a light guide or totally reflected, depending on how the ver Ratio of refractive indices or densities in the light guide and is in the surrounding medium. A distinction is made here Lichen arrangements of the effect exploited by the environment a part of the light guide with a dense medium ever increasing amount of light is coupled out of the light guide is, whereby analog measurements, for example a fill befitting, should be possible. A disadvantage of the known Setup is, however, that the measurement effect is theoretical exists, but is extremely small in practice and therefore exact measurements are hardly possible, especially since only one Analog signal is available.

Finally, from DE-OS 21 55 049 there is still an optical one Comparative device with optical fibers known. At this known device is, as already to the next The embodiment described above is explained in the axial Distance from an end face of one exposed to light Optical fiber one with a reflective surface  provided reference plane, the axial distance to be measured from the end face of the light guide. For this purpose, several receptions are received radially next to the light guide arranged light guide, the signal each individual optical detectors is supplied. This optical detec gates are in turn connected to an electronic analog circuit connected. In this known device you do take advantage of the fact that with increasing Ab stood the reflective reference surface from the front surface the edge of the reflected beam of light always radially from the end face of the light guide wanders and one after the other the end faces of the various which radially spaced receiving light guide sweeps. As a result, a regionally linear output signal can can only be removed from the receiving light guide, whose end face is just from the edge of the reflected beam lenbündels is swept. This makes the well-known Take advantage of the device by using the straight "linear" Signal to form an overall broader linear range is used. A disadvantage of this known device is therefore also only for a single measurement is suitable and a digital output value of the signal is also not available here.

The invention is based on the object, one To continue sensor of the type mentioned that he is suitable for a variety of measurement tasks and The problem with the adjustment is that local Do not make disturbances in the vicinity of the sensor noticeable and that ultimately, above all, a continuous digital display of the respective output signal is possible.  

This object is achieved in that the Body is designed as an elongated light guide, in the light rays are totally reflected several times, that the optical refractive index of the light guide over its length towards the end face decreases that the photosensitive Elements at an axial distance from an end face are arranged and an impact surface for one from the Forming emerging bundles of light rays at the end face, and that the elements to an evaluation circuit with a counter connected to the number of be from the bundle illuminated or alternatively the non-illuminated elements outputs as an output signal in digital form. Below only the count of the illuminated elements will be closer seeks. Should the number of non-illuminated elements ge counts and evaluated, the total number of all Elements to be known.

The object on which the invention is based is thus full come solved. When using an elongated light on the one hand Adjustment problems avoided because the length of the Seen light guide uniform light beam ver training relationships, and in the vicinity of the Any existing discontinuities in this way averaged. By guiding the light rays in the light conductors can solve numerous different measuring tasks will. This enables continuous level measurements and geometric sizes, in particular lengths, can also be used can be measured in this way. This list limits however, the scope of the invention is by no means limited.  

Another important advantage of the invention is that the Angular range of the bundle emerging from the light guide, in other words the so-called "acceptance cone" in digi taler form is measured by the number of at a certain value of physical quantity illuminated ele elements are counted and displayed. Any aging phenomena or other drift phenomena have an effect so not disturbing the measurement result because the off value circuit for each individual photosensitive element only makes a yes / no decision, so that when appropriate set trigger level for the respective element without What is important is how great the intensity of each hit is light beam is or how the conversion factor from incident light beam to emitted voltage in the Element itself changed due to signs of aging Has.

Overall, the invention thus sets a universal settable, robust, reliable and against aging apparently insensitive sensor available.

One area of application of the sensor according to the invention is the physical quantity a level of a liquid. In In this case, the light guide is submerged over part of it axial length in the liquid and the optical refraction index of the light guide decreases from its lower end from the top. More generally, the physical one Size the location of a boundary layer between two media, e.g. also between liquids, with different refractions indices.  

This measure has the advantage that continuous filling level measurements with digital measured value generation are possible, because the opening angle of the acceptance cone of each lowest refractive index is determined, the ge described gradients of the refractive index over the axial Length of the light guide is just the one who is on the Surface of the surrounding liquid. With others Words, the opening angle of the acceptance cone is one maximum size with minimum fill level continuously smaller up to a maximum level, whereby this Varia tion of the acceptance cone in the manner described in one digital measured value is transformed.

In a variant of this embodiment, the Light guide dimensioned so that the product is half its thickness and the tangent of the critical angle of the total reflection of the fiber optic material lying outside the measuring liquid to the surrounding medium much smaller, preferred is 1/3 to 1/40 of the length of the light guide.

This measure has the advantage that at extremely low a large jump in signal levels when the Level just exceeds or falls below a level that the product of half the thickness of the light guide and the tan corresponds to the mentioned limit angle. Up to this The level of the level is namely the opening angle of the Acceptance cone a very large value, the critical angle Light guide / surrounding medium outside the liquid (in general: air), while when exceeded this height the opening angle abruptly on a very much lower value that decreases the critical angle Lichtlei term material / liquid corresponds. This signal jump is  in the light guide materials under discussion here much larger than the jumps in sections visually graded light guide, such as this was described in the previous section. The very large signal jump can therefore be advantageous as a "reserve display ge "can be used to signa the user of the sensor lize that the level to a very low lower Limit has dropped.

In a further field of application of the invention physical size has a length and the light guide light-emitting elements, which after irradiation by means of a primary light emit secondary light, the bundle Beams of light in the radial direction at a distance of length from the face meets the light guide on the side.

This embodiment of the invention, which is just if making use of the central idea of the invention, the variation of the acceptance cone in a digital way Capture thus has the advantage that a length touching can be measured because, depending on the refractive index of the luminescent light guide area on which the radial Measuring beam falls, at the front end a beam emerges, the acceptance cone of the refractive index of the depends on the light guide area.

The measures also have the advantage that instead of one diffuse light into the light guide also a Irradiation is possible using a parallel bundle, then the diffuse required for the invention Light rays can be represented by the secondary light. This can happen, for example, that the primary  light hits impurities (color centers) in the light guide, that diffusely reflecting interfaces on which the primary light meets, are provided or that the light guide with Luminescence centers are provided, which in turn are secondary generate light.

In a further development of this variant, the light guide ter according to the invention axially juxtaposed sections different optical refractive index.

This measure has the advantage that the light guide is simple cher can be made because for the sections on existing materials can be used.

In embodiments of the invention, the light is there ladder made of glass or plastic, e.g. Polymethyl acrylate (PMMA).

This measure also has the advantage that known mate materials with also known reproducible properties ten can be used.

In another embodiment of the invention Impact surface inclined to the axis of the light guide.

This measure has the advantage that when the acceptance is varied dance cone due to the inclination of the impact surface a ver enlargement of the area in which the edge of the Acceptance cone fluctuates.

In a preferred embodiment of this variant, the Impact surface also curved in a predetermined manner  fen, in this way any non-li compensate for the sensor's localities.

Finally, there is another embodiment of the invention preferred, in which the light source on a pulse generator is connected and the evaluation circuit a difference has formers, the inputs of which are the measured values at switched or switched off light source can be supplied.

This measure has the advantage that measurements in the Impulse pauses measured and thus external light influences can be compensated.

Further advantages result from the description and the attached drawing.

It is understood that the above and the features explained below not only in each specified combination but also in other combinations can be used alone or without the frame to leave the present invention.

Embodiments of the invention are in the drawing are shown and are described in the following description explained in more detail. Show it:

Figure 1 is a schematic representation of an optical fiber for explaining the present invention.

FIG. 2 shows a beam path as it occurs in the light guide according to FIG. 1;

Fig. 3 shows a first embodiment of a surface measuring arrangement for use in the sensor according to the invention;

FIG. 4 shows a variant of the exemplary embodiment according to FIG. 3 with a linear light measuring arrangement;

Figures 5a and 5b an embodiment of an inventive SEN sensor for measuring a level with associated characteristic of the refractive index over the length of the optical fiber used.

Fig. 6 is a variant of the embodiment according to Figure 5a stepped light guide.

Fig. 7a to 7c is a detailed representation of the sensor of Figure 5a or Figure 6, illustrating a possible according OF INVENTION dung reserve indicator..;

Fig. 8 shows an embodiment of an inventive sensor for measuring a length;

Figure 9 is a highly schematic circuit diagram for explaining the connection of a SEN according to the invention sensor.

Fig. 10 shows a further variant, similar to FIG. 1, for the measuring sensitivity raised stabili hung a SEN according to the invention the sensor.

In Fig. 1, 1 denotes a light source from which a diverging beam of rays 2 emerges and enters an adjacent upper end face 3 of a cylindrical light guide 10 . The punctiform light source 1 with the diverging rays bundle 2 is only exemplary to understand here, it will be explained further below that parallel rays can also be used, from which divergent secondary light is derived inside the light guide.

In Fig. 1, 11 shows a first, axially directed light beam which passes through the light guide 10 without further deflection or obstruction. With 12 a , 12 b , a second light beam is marked, which hits flat on a Mantelflä surface 19 of the light guide 10 that it is totally reflected. The light beam 12 a , 12 b thus continues its path through the light guide 10 in the axial direction. 13 a , 13 b , on the other hand, denotes a third light beam which strikes the lateral surface 19 at such a steep angle that it is coupled out of the light guide 10 .

This means in the result that after several reflections in the light guide 10 only light rays 11 or 12 a , 12 b are guided, which are either strictly axially or so flatly directed that they are totally reflected on the lateral surface 19 . This creates a so-called “acceptance cone” 14 , which is used to denote the shape of a diverging bundle 17 of light rays emerging from a lower end face 16 .

An impact surface 15 is defined at an axial distance h from the lower end face 16 . If one designates the opening angle of the acceptance cone 14 with β , then there is a circular light surface with a circumferential ring area of radial width x , which depends on the angle β and the axial distance h , in the impact surface 15 with a circular lower end face 16 .

Now changes due to a change in the refraction conditions in the light guide 10 or in the surrounding medium, the critical angle of the total reflection, also changes the angle β and thus the dimension x .

FIG. 2 shows the relationships explained for FIG. 1 again for the quantification of the effect that occurs. A fourth light beam 18 is now guided in the light guide 10 , the section 18 a of which strikes the outer surface 19 just under the critical angle α _T of total reflection. Expressed in graphic terms, this means that all the light incident in the hatched area in FIG. 2 radiates total reflection, while all rays incident more steeply than the light beam 18 are coupled out of the light guide 10 . The light beam 18 is now (just) still totally reflected in its section 18 a and a reflected section 18 b strikes the lower end face 16 . Provided that the light beam section 18 b occurs outside the total reflection area of the refractive ratio se prevailing on the lower end face 16 , a section 18 c of the light beam 18 is coupled out of the lower end face 16 , namely at an angle β which is just the opening angle β of the acceptance cone 14 in FIG. 1 corresponds.

Denoted by n _i the refractive index of the light guide 10 , with n _a the refractive index of the medium surrounding the light guide 10 in the region of its lateral surface 19 and with n _st the refractive index of the medium surrounding the light guide 10 on the lower end face 16 , one can show gene that the following applies to the opening angle β of the acceptance cone 14 :

It goes without saying that n _{i is} greater than n _a and n _st . In the event that the refraction ratios on the lateral surface 19 and on the end surface 16 are the same, ie n _a = n _st , the formula given is simplified accordingly.

It can thus be seen that the width x to be seen in FIG. 1, via the opening angle β and the distance h, is directly a measure of the refraction ratios of the light guide 10 to the medium surrounding it.

According to the invention it is now provided to output the width x in digitized form as a measured value.

Fig. 3 shows a this embodiment of the invention, can be seen at the turn of the optical fiber 10 from which a light beam in the shape of the acceptance cone 14 emerges below. In addition, an acceptance cone 14 a is drawn in dashed lines to symbolize a second measured value.

Below the light guide 10 , a flat detector array 22 can be seen at an axial distance in the imaginary impingement surface, which can be designed, for example, as a charge shift semiconductor component (CCD) . The detector array 22 consists of a multiplicity of detector elements 23 distributed in an area which can be individually controlled and read out. A symbolized data line 24 leads to an evaluation circuit 25 , which essentially contains a digital counter.

In the illustrated example of the solidly drawn acceptance cone 14 , the hatched in Fig. 3 8 detector elements are illuminated, so that after counting these elements via the data line 24 in the evaluation circuit 25, a digital value "8" is output at the output thereof. One can achieve an almost arbitrary resolution of the measurement result by a corresponding plurality of detector elements 23 and, if this should be of practical importance, by setting a certain trigger threshold for only partially illuminated detector elements 23, a limit value from which on Detector element 23 is counted as illuminated or non-illuminated. Any gray transitions in the edge area of the acceptance cone 14 can also be precisely defined in this way.

It is seen from Fig. 3 readily appreciate that at a magnification 14 fication of the acceptance cone in a cone 14 a (shown in dashed lines) has a correspondingly larger number of Detek gate elements illuminated 23 and thus a correspondingly larger digital value appears at the output of the evaluation circuit 25th

FIG. 4 shows a variant in which the distance x from FIG. 1 is not determined by an area measurement as in FIG. 3 but only by a measurement along a straight line. For this purpose, a linear detector array 26 is provided, for example a linear diode gate or the like. It can be seen from Fig. 4 that in the case of the solidly drawn acceptance cone 14 four detector elements 23 are illuminated, which in turn have been hatched in Fig. 4, while when opening the acceptance cone to a value 14 a in the illustrated example, six detector elements be illuminated. In this case too, the number of illuminated detector elements is counted and output at the output of the evaluation circuit 25 in the form of a digital value.

With a further group of exemplary embodiments of the invention, as shown in FIGS. 5 to 7, it is possible to measure the fill level of a liquid with a sensor of the type according to the invention.

Fig. 5a shows an elongated light guide 50 which dips part of its axial length in the measuring liquid 33 , which in turn is contained in the container with the walls 31 , 32 . The light guide 50 is in turn hen on its lower end face 16 with a mirror 36 verses.

A special feature of the arrangement according to FIG. 5a is that the refractive index n of the light guide 50 varies over its axial length z . The course of the refractive index n over the axial length z is shown in FIG. 5b and it can be seen that the refractive index n assumes its highest value at approximately 1.6 at the lower end and its lowest value at approximately 1.4 at the upper end .

It has already been explained in the basic explanations of the mechanism of action of the sensors according to the invention in FIGS . 1 and 2 that the opening angle β of the acceptance cone 14 is smaller, the lower the refractive index of the light guide material. Illustrated in a clear manner, this means that with a low refractive index of the light guide material, more and more light beams are coupled out of the light guide and only the very flat, ie, almost parallel to the light guide axis, light beams are guided in the light guide. In the event that the refractive index varies over the length of the light guide, this means that the area of the light guide limits and thus defines the acceptance cone which has the lowest refractive index.

In the case of the exemplary embodiment in FIGS . 5a and 5b, however, in relation to the surrounding measuring liquid 33 , this is always the area of the light guide 50 which adjoins the surface of the liquid 33 , ie defines a fill level 51 .

If, in the illustration in FIG. 5a, the fill level 51 drops from an upper maximum value to a lower minimum value, this means that an acceptance cone emerging at the not shown upper end of the light guide 50 increases its opening angle β with decreasing fill level 51 , so that a digital level measurement is possible in the manner described with reference to FIGS. 3 and 4.

In the case of the light guide 50 with the refractive index gradient according to FIG. 5b, this characteristic can be generated by setting the degree of polymerization over the length, for example in the case of a plastic light guide. A change in the refractive index via selective Druckein effect, radiation or the like is also conceivable.

Instead of a light guide 50 with a continuously varying refractive index n, it is also possible, as shown in FIG. 6, to use a light guide 52 which is divided into a plurality of axially adjacent sections 53 . Sections 53 ₀ , 53 ₁ . . . 53 _n are formed so that their associated refractive indices n ₀ , n ₁ , n ₂ ... n _n decrease from bottom to top. The characteristic curve of FIG. 5b for the light guide 50 of FIG. 5a would therefore tend to be the same, but in a slightly stepped form.

The sections 53 can also be provided with a continuously varying refractive index n , so that the grading of the sections 53 enables a coarse measurement and, through their axially varying refractive index n , a fine measurement within that section 53 is also possible.

FIGS. 7a to 7c also show a phenomenon which occurs in the light guides 50 according to FIG. 5a, 5b or 52 according to FIG. 6 and which is of particular advantage in order to indicate small residual amounts of measuring liquid 33 . This is particularly advantageous for example in the level indicator in gasoline tanks of motor vehicles if a "reserve indicator" is to be set up as a particularly prominent value in order to draw the driver's attention to the fact that the gasoline supply has dropped below a certain minimum amount.

To explain this phenomenon in FIGS. 7a to 7c, reference is first made to FIG. 7c, where a light guide 59 of width d is shown with its longitudinal axis 64 . At the lower end of the light guide 59 , the two critical angles of the total reflection α _TL for the case of air surrounding the light guide 59 and a _TF for the case of the measuring liquid 33 surrounding the light guide 59 are shown.

In FIG. 7a, 60 denotes a light beam entering from above, which is inclined so that it runs just under the critical angle α _TL for surrounding air and is thus totally reflected on the outer surface 19 of the light guide 59 surrounded by air. At the very low fill level 61 shown in FIG. 7 a, this means that - although a very small lower part of the light guide 59 is still surrounded by measuring liquid 33 - this light beam 60 is reflected upwards again via the mirror 36 and the lateral surface 19 and thus defines an acceptance cone at the upper end of the light guide 59 , the opening angle of which defines the limiting angle α _TL for surrounding air, ie is very large.

This reflection of light rays 60 with an inclination up to the critical angle α _TL for surrounding air is possible until the fill level 61 has reached a height 62 shown in FIG. 7a. The height 63 results from the line of intersection of a cone 63 about the axis 64 of the light guide 59 , the outer angle of the cone 63 being exactly the same as the critical angle α _TL for surrounding air. If the fill level exceeds the height 62 , reflection of light rays 60 towards the upper end of the light guide 59 is no longer possible. Rather, it occurs in Fig. 7c marked case that at a higher fill level 65 the light beam 60 is coupled out into the measuring liquid 33 because now the larger critical angle α _TF for surrounding liquid defines the refraction conditions on the outer surface 19 of the light guide 59 .

If one now considers the characteristic of the opening angle β of the acceptance cone as a function of the filling height F shown in FIG. 7b, it can be seen that up to the height 62 the opening angle β assumes the value β _L , which - as already explained above - from Limit angle a _TL is defined for surrounding air. If the fill level exceeds the height 62 , the value of the opening angle β suddenly drops to a value β ₀, which is defined by the critical angle α _TF for surrounding liquid.

If the light guide, as shown in Fig. 6 with 52, is stepped in its axial length, then, as Fig. 7b shows at the upper edge, further stages β ₁ etc. can follow, but these further stages are much smaller than that lower level from b _L to β ₀ because such large jumps in limit values α _T no longer occur.

The very large signal jump from b _L to β ₀ can therefore be used to activate a reserve display. The point of use of this reserve indicator can be determined, as can easily be seen in FIG. 7a, by dimensioning the thickness d of the light guide 59 in relation to the critical angle α _TL for surrounding air.

Another area of application for sensors according to the invention is the measurement of geometric variables, in particular a length y , as is shown in FIG. 8 using an example.

A light guide 70 is divided in the axial direction into sections 71 , one of which is denoted by 71 _n in FIG. 8. The sections 71 consist of luminescent material and one of the luminescent elements is denoted by 72 in section 71 _n . The sections 71 each have different refractive indices and the refractive index of the section 71 _n is denoted by n _n . The value of the refractive index increases from section to section in Fig. 8 from right to left.

A narrow beam or a light beam 73 , which is directed radially to the light guide 70 , strikes a side surface 74 of the light guide 70 . As a result, luminescence is excited in each of the sections 71 and the secondary light emitted here by the luminescence element 72 propagates in the axial direction of the light guide 70 . 8 to the left in FIG. 8 arrives at the end of the light guide 70 on a radial face 75 and exits there again in the form of an acceptance cone 14 , so that a measurement of the opening angle β _{n of} the acceptance cone 14 by means of the manner already described of a detector array 22 is possible.

Because the refractive index n of the sections 71 increases toward the end face 75 , the opening angle of the acceptance cone 14 is determined by the section 71 itself acted upon by the light beam 73 , because the sections 71 lying further forward in the beam direction towards the end face 75 always have a larger acceptance cone Allow 14 , but do not take advantage of this due to the lack of suitable "steep" jets.

It can therefore be determined by measuring the opening angle β _n , on which of the sections 71 the light beam 73 has fallen. In other words, this means that the opening angle b _{n is} a measure of the length y if y is defined as the distance of the light beam 73 from the front face 75 .

It is understood that in this way also correspond appropriate expansion of the arrangement in the sensors plane can be created where the position of one on can be measured in the plane.

Fig. 9 shows a very schematic circuit diagram of a circuit arrangement for operating a sensor according to the invention.

A pulse generator 100 operates the light source 1 , which, on the basis of this, emits a clocked beam 2 onto a light guide 102 . External light 103 also falls on the light guide 102 . The light guide 102 is connected via suitable detector and evaluation devices, as were explained in FIGS. 3 and 4, to an amplifier 104 , which is also struck by the output of the pulse generator 100 .

The purpose of the circuit arrangement according to FIG. 9 is to regulate the influence of the external light 103 . For this purpose, the signal picked up by the light guide 102 and originating solely from the external light 103 is determined and stored in the amplifier 104 during the pulse pauses of the pulse generator 100 . During a pulse of the pulse generator 100 , a measured value is again formed, which has arisen in the light guide 102 by the beam 2 of the light source 1 and the influence of the external light 103 and the previously determined measured value of the external light 103 alone is subtracted from this measured value. Since the external light 103 is generally a constant disturbance variable, the influence of the external light 13 can be compensated for in this way.

Fig. 10 finally shows another way to achieve a Ver enlargement of the width x in Fig. 1 to increase the accuracy by increasing the resolution.

A light guide 106 similar to that in FIG. 1 is located with its lower end at an axial distance from a striking surface 107 , which, however, is inclined to a longitudinal axis 108 of the light guide 106 . In this way, there is an increased width x 'when the bundle of light rays len falls in the shape of the acceptance cone 14 on the impact surface 107 .

It is also shown in Fig. 10 with 107 a , 107 b that one can give the impingement surface 107 in addition to or instead of the inclination to the axis 108 also a curved course in order to compensate or generate certain characteristics in this way, depending according to how it is desirable in the specific application.

Claims (9)

1. Sensor for converting a physical quantity into an electrical output signal, with a light source ( 1 ), from which a bundle ( 2 ) of light beams ( 11 , 12 , 13 ; 18 ; 60 ; 73 ) is coupled into a light-conducting body, wherein the light rays ( 11 , 12 , 13 ; 18 ; 60 ; 73 ) are totally reflected at an interface ( 16 , 19 ) of the body depending on the physical size or are coupled out of the body and the totally reflected light rays ( 12 b ; 18 b ; 60 ) fall on an end face ( 16 ), and with a plurality of light-sensitive elements ( 23 ) for detecting an angular range β taken up by the bundle ( 2 ) after total reflection or decoupling, characterized in that the body as an elongated light guide ( 10 ; 50 ; 52 ; 59 ; 70 ; 102 ; 106 ) is formed, in which light beams ( 12 b ; 18 b ; 60 ) are repeatedly totally reflected that the optical refractive index ( s ) of the light guide ( 50 ; 52 ; 59 ) over its length decreases towards the end face, that the light-sensitive elements ( 23 ) were in an axial distance ( h ) from the end face ( 3 ; 16; ) are arranged and form a striking surface ( 15 ; 107 ) for a beam ( 17 ) emerging from the end face ( 3 ; 16; ), and that the elements ( 23 ) are connected to an evaluation circuit ( 25 ) with a counter, which outputs the number of elements ( 23 ) illuminated by the bundle ( 17 ) or, alternatively, the non-illuminated elements ( 23 ) as an output signal in digital form. 2. Sensor according to claim 1, characterized in that the physical quantity is a fill level ( 51 ; 61 , 65 ) of a medium, preferably a liquid ( 33 ), that the light guide ( 50 ; 52 ; 59 ) over part of its axial length immersed in the medium and that the optical refractive index ( n ) of the light guide ( 50 ; 52 ; 59 ) decreases from its lower end upwards. 3. Sensor according to claim 2, characterized in that the light guide ( 59 ) is dimensioned such that the product of its half thickness d and the tangent of the critical angle α _TL the total reflection of the light guide material lying outside the medium to the medium surrounding it is substantially smaller, preferably a third to a fortieth of the length of the light guide ( 59 ). 4. Sensor according to one of claims 1 to 3, characterized in that the physical size is a length ( y ), that the light guide ( 70 ) has light-emitting elements ( 72 ) which emit secondary light after irradiation by means of a primary light, and that the bundle of light rays ( 73 ) in the radial direction at a distance of length ( y ) from the end face ( 75 ) since Lich strikes the light guide ( 70 ). 5. Sensor according to claim 4, characterized in that the light-emitting elements are luminescent elements ( 72 ). 6. Sensor according to one of claims 1 to 5, characterized in that the light guide ( 70 ) in the axial direction arranged side by side sections ( 71 ) with different optical refractive index n on. 7. Sensor according to one of claims 1 to 6, characterized in that the light guide ( 10 ; 50 ; 52 ; 59 ) consists of glass or plastic. 8. Sensor according to one of claims 1 to 7, characterized in that the light source ( 1 ) is connected to a pulse generator ( 100 ) and that the evaluation circuit ( 25 ) has a difference former ( 104 ), the inputs of which are the measured values can be supplied when the light source ( 1 ) is switched on or off. 9. Sensor according to one of claims 1 to 8, characterized in that the impact surface ( 107 a , 107 b ) is curved in a predetermined manner.